With the inception of the tractor-trailer vehicles, the tractor vehicle was relatively low to the ground as compared to the trailer unit which was high and boxy to include as much volume as the law would permit for the carrying of cargo. As a result, the air flow impinging on the trailer at high speeds exerts a high degree of force, particularly if a strong head wind existed. Streamline panels or structures have been developed for tractor cab mounting to reduce this airflow drag. This, however, coupled with long downhill grades can be a dangerous situation because more braking effort is now required to slow down or stop these vehicles than was required before the streamline panel improvements were made.